There is a great desire to increase the efficiency of electrical motors for saving energy and money. In industrialized countries, electric motors consume over 50% of electricity production. In the U.S., an increase in motor efficiency of only 1% would result in an electrical energy cost savings of over $1 B annually.
One way to increase the efficiency of electrical motors is to change their design and construction to reduce energy losses. Air core motors, which locate the windings within a magnetic airgap instead of within slots in a steel stator, can increase efficiency by reducing or eliminating magnetically induced losses. But despite the increased efficiency of air core motors over conventional motors, in many applications, especially fixed speed applications, the efficiency improvement is not as high as desired.
Although air core motors can provide higher efficiency than conventional induction motors, with our dynamometer test results to date approaching 99.0% efficiency, they require a variable speed drive (VSD) to operate. The VSD is required for operation of air core motors because they are synchronous machines. VSD efficiency is typically between 92–96% efficient, thereby unfortunately reducing significantly the total possible efficiency of the motor system. In addition, air core motors also have a much lower armature winding inductance, up to 1000 times lower. This low inductance, in many cases, requires the addition of inductors to reduce the occurrence of harmonic causing ripple currents caused by the high frequency switching regulation of the VSD. These inductors add additional losses to the motor system. Although the air core motor with VSD provides a much higher efficiency than a comparable induction motor with VSD for variable speed applications, the efficiency advantage is not as significant as desirable when comparing against an induction motor running directly from line power for fixed speed applications.
Unlike conventional slot wound synchronous motors, air core motors also cannot line start. Conventional slot wound synchronous motors can forego the use of a VSD and can line start by utilizing an Amortisseur winding on the rotor that allows asynchronous starting and operation up to synchronization speed. In contrast, air core motors inherently utilize a much larger, as much as 30 times larger, magnetic airgap for the locating of the armature windings. The location of the windings in the large airgap provides the desired efficiency increase. However, the large magnetic airgap also results in the field flux being generated almost exclusively by permanent magnets on the rotor that drive the flux across the armature airgap. As a result, the armature windings do not generate a significant external magnetic field to excite an Amortisseur winding, if it were included on the rotor. An additional problem of conventional air core motor systems is the high pole count for increasing the efficiency and reducing motor size and costs, which would result in an operating speed, if it could be operated line synchronously, that is much too low, i.e. only 300 rpm for typical 24 pole air core motor. This low speed makes it both incompatible for typical motor applications and it also substantially reduces the power output capability for a given motor size and cost, making it uncompetitive.
Despite the increased efficiency of air core motors over conventional motors, in many applications, especially fixed speed applications, the efficiency improvement is not as high as desired. A new motor system for providing further increased efficiency is needed. Thus, there has long been a need for a new motor system for providing further increased efficiency.